An 18-year-old female player presents with a six-year history of recurring ‘locking’ episodes in her left knee. Approximately once per month she reported a sense that her knee ‘locked’ and that she had to physically manipulate something ‘back into place’. On examination there was a tender bony prominence around the distal medial thigh. The knee examination was otherwise normal with no joint line tenderness, no effusion and a normal knee range of motion.
Both the x-ray and MRI show a thin osteochondroma arising from the medial distal femoral metadiaphyseal region. This measures about 2cm in length with a 1cm base. The osteochondroma shows typical medullary continuity with the femoral shaft and there is a thin cartilage cap. The exostosis bulges into the distal vastus medialis muscle belly. It is surrounded by feathery oedema within the muscle belly. The medial meniscus is normal, as is the remainder of the medial compartment. There is no other clear cause for her mechanical symptoms.
This player opted to have the osteochondroma excised. This was done without complication. She was able to return to football approximately two months following this surgery and has not had any further problems. The mechanical symptoms have completely resolved.
‘Locking’ is a frequently reported symptom. In most cases patients describe a sense of something clicking and catching within their knee. These symptoms are ‘pseudo-locking’ and are generally due to pain or maltracking. True locking occurs when the knee joint gets physically stuck into one position and cannot move, and is far less common. It is important to get a good feeling for what your patient is actually describing to you. In most cases true locking reflects a more significant structural problem and requires specific treatment. This case illustrates a rather unusual cause of locking. Other cases of locking include an unstable meniscal tear, a loose body (following a chondral or osteochondral injury) or unstable surgical implants.
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